A fundamental challenge associated with the administration of medications to patients in need thereof is the relationship between tolerability and efficacy. The therapeutic index is the range between which an efficacious dose of a substance can be administered to a patient and a dose at which undesired side effects to the patient are noted. Generally, the larger the difference between the efficacious dose and the dose at which side effects initiate, the more benign the substance and the more likely it is to be tolerated by the patient.
Heart failure, particularly congestive heart failure (CHF), one of the leading causes of death in industrialized nations. Factors that underlie congestive heart failure include high blood pressure, ischemic heart disease, exposure to cardiotoxic compounds such as the anthracycline antibiotics, radiation exposure, physical trauma and genetic defects associated with an increased risk of heart failure. Thus, CHF often results from an increased workload on the heart due to hypertension, damage to the myocardium from chronic ischemia, myocardial infarction, viral disease, chemical toxicity, radiation and other diseases such as scleroderma. These conditions result in a progressive decrease in the heart's pumping ability. Initially, the increased workload that results from high blood pressure or loss of contractile tissue induces compensatory cardiomyocyte hypertrophy and thickening of the left ventricular wall, thereby enhancing contractility and maintaining cardiac function. Over time, however, the left ventricular chamber dilates, systolic pump function deteriorates, cardiomyocytes undergo apoptotic cell death, and myocardial function progressively deteriorates.
Neuregulins (NRGs) and NRG receptors comprise a growth factor-receptor tyrosine kinase system for cell-cell signaling that is involved in organogenesis and cell development in nerve, muscle, epithelia, and other tissues (Lemke, Mol. Cell. Neurosci. 7:247-262, 1996 and Burden et al, Neuron 18:847-855, 1997). The NRG family consists of four genes that encode numerous ligands containing epidermal growth factor (EGF)-like, immunoglobulin (Ig), and other recognizable domains. Numerous secreted and membrane-attached isoforms function as ligands in this signaling system. The receptors for NRG ligands are all members of the EGF receptor (EGFR) family, and include EGFR (or ErbB1), ErbB2, ErbB3, and ErbB4, also known as HER1 through HER4, respectively, in humans (Meyer et al., Development 124:3575-3586, 1997; Orr-Urtreger et al., Proc. Natl. Acad. Sci. USA 90: 1867-71, 1993; Marchionni et al., Nature 362:312-8, 1993; Chen et al., J. Comp. Neurol. 349:389-400, 1994; Corfas et al., Neuron 14:103-115, 1995; Meyer et al., Proc. Natl. Acad. Sci. USA 91:1064-1068, 1994; and Pinkas-Kramarski et al., Oncogene 15:2803-2815, 1997).
The four NRG genes, NRG-1, NRG-2, NRG-3, and NRG-4, map to distinct chromosomal loci (Pinkas-Kramarski et al., Proc. Natl. Acad. Sci. USA 91:9387-91, 1994; Carraway et al., Nature 387:512-516, 1997; Chang et al., Nature 387:509-511, 1997; and Zhang et al., Proc. Natl. Acad. Sci. USA 94:9562-9567, 1997), and collectively encode a diverse array of NRG proteins. The gene products of NRG-1, for example, comprise a group of approximately 15 distinct structurally-related isoforms (Lemke, Mol. Cell. Neurosci. 7:247-262, 1996 and Peles and Yarden, BioEssays 15:815-824, 1993). The first-identified isoforms of NRG-1 included Neu Differentiation Factor (NDF; Peles et al., Cell 69, 205-216, 1992 and Wen et al., Cell 69, 559-572, 1992), heregulin (HRG; Holmes et al., Science 256:1205-1210, 1992), Acetylcholine Receptor Inducing Activity (ARIA; Falls et al., Cell 72:801-815, 1993), and the glial growth factors GGF1, GGF2, and GGF3 (Marchionni et al. Nature 362:312-8, 1993).
The NRG-2 gene was identified by homology cloning (Chang et al., Nature 387:509-512, 1997; Carraway et al., Nature 387:512-516, 1997; and Higashiyama et al., J. Biochem. 122:675-680, 1997) and through genomic approaches (Busfield et al., Mol. Cell. Biol. 17:4007-4014, 1997). NRG-2 cDNAs are also known as Neural- and Thymus-Derived Activator of ErbB Kinases (NTAK; Genbank Accession No. AB005060), Divergent of Neuregulin (Don-1), and Cerebellum-Derived Growth Factor (CDGF; PCT application WO 97/09425). Experimental evidence shows that cells expressing ErbB4 or the ErbB2/ErbB4 combination are likely to show a particularly robust response to NRG-2 (Pinkas-Kramarski et al., Mol. Cell. Biol. 18:6090-6101, 1998). The NRG-3 gene product (Zhang et al., supra) is also known to bind and activate ErbB4 receptors (Hijazi et al., Int. J. Oncol. 13:1061-1067, 1998).
An EGF-like domain is present at the core of all forms of NRGs, and is required for binding and activating ErbB receptors. Deduced amino acid sequences of the EGF-like domains encoded in the three genes are approximately 30-40% identical (pairwise comparisons). Further, there appear to be at least two sub-forms of EGF-like domains in NRG-1 and NRG-2, which may confer different bioactivities and tissue-specific potencies.
Cellular responses to NRGs are mediated through the NRG receptor tyrosine kinases EGFR, ErbB2, ErbB3, and ErbB4 of the epidermal growth factor receptor family. High-affinity binding of all NRGs is mediated principally via either ErbB3 or ErbB4. Binding of NRG ligands leads to dimerization with other ErbB subunits and transactivation by phosphorylation on specific tyrosine residues. In certain experimental settings, nearly all combinations of ErbB receptors appear to be capable of forming dimers in response to the binding of NRG-1 isoforms. However, it appears that ErbB2 is a preferred dimerization partner that may play an important role in stabilizing the ligand-receptor complex. ErbB2 does not bind ligand on its own, but must be heterologously paired with one of the other receptor subtypes. ErbB3 does possess tyrosine kinase activity, but is a target for phosphorylation by the other receptors. Expression of NRG-1, ErbB2, and ErbB4 is known to be necessary for trabeculation of the ventricular myocardium during mouse development.
Neuregulins stimulate compensatory hypertrophic growth and inhibit apoptosis of myocardiocytes subjected to physiological stress. In accordance with these observations, administration of a neuregulin is useful for preventing, minimizing, or reversing congestive heart disease resulting from underlying factors such as hypertension, ischemic heart disease, and cardiotoxicity. See, e.g., U.S. Pat. No. 6,635,249, which is incorporated herein in its entirety.
In view of the high prevalence of heart failure in the general population, there continues to be an unmet need to prevent or minimize progression of this disease, such as by inhibiting loss of cardiac function or by improving cardiac function